There are a number ways to control message size in Exchange. Many actually. Even down to the header size.


Here are the options I have been most likely to use:
This is where you set your server parameters. This setting applies to all users on the server:
To view your current settings:

Get-TransportConfig | Select MaxSendSize,MaxReceiveSize

To modify your settings:

Set-TransportConfig -MaxSendSize 15MB -MaxReceiveSize 15MB

Here is where you can set the parameter based on a receive connector only:

Get-RecieveConnector | Select MaxMessageSize

To modify:

Set-ReceiveConnector -Identity “receiveconnectorname” -MaxMessageSize 15MB

And this for a send connector:

Get-SendConnector | Select MaxMessageSize

To modify:

Set-SendConnector -Identity “sendconnectorname” -MaxMessageSize 15MB

This is where you would set it for the individual mailbox. So, you might want have your other settings high, and restrict by mailbox if the need is there.
To view a mailbox’s limits:

Get-Mailbox | Select Name,MaxSendSize,MaxReceiveSize
Get-Mailbox mailboxname | Select Name,MaxSendSize,MaxReceiveSize

To modify:

Set-Mailbox jgz -MaxSendSize 100MB -MaxReceiveSize 100MB